The concept of computer-assisted stereotactic methods effectively began in 1979. By 1996 it was generally accepted that volumetric stereotactic procedures were feasible including the use of stereotactically directed instruments with respect to pre- or intraoperative displayed images.
Computer assisted image guided stereotactic surgery is beneficial in the biopsy and ablation of primary brain tumors, benign and malignant, as well as in many other intracranial procedures using computed tomography, MRI, positron emission tomography (PET) and single photon emissions tomography (SPECT). It is especially useful in accurate localization of intracranial vital structures. The passive articulated arm has been shown to be useful in the resection of brain metastases. Surgical navigation is used in head and neck tumors, employing MRI and CT imaging. Stereotactic interstitial brachytherapy has been used for inoperable head and neck cancers, with a head holder used for immobilization. Brachytherapy, the insertion of an ablative radioactive material into an otherwise inoperable tumor, can be placed accurately and through a smaller incision using computer assisted image guided stereotactic surgery. Other uses include vaporization of tumor slice and MRI, digital angiography, and computer-resident stereotactic atlases. Such methods are particularly utilized in neurosurgical and otolaryngological procedures of the head and orthopaedic procedures of the pelvis and spine.
The insertion of pedicle screws for spine fusion procedures is enhanced by computer-assisted methods. At first, 3-D images from CT scans were used but these have been replaced by computer-assisted fluoroscopy. For the insertion of iliosacral screws for pelvic ring disruption, the use of CT images has been shown to be accurate and safe, and can be employed when conventional X-ray is not useful due to the presence of contrast media in the bowels, or anatomic variations resulting in a narrow passage for the screw.
An essential capability of and step in the use of computer assisted surgery is registering the computer system and the digitized CT or MRI image data set to the patient in a common frame of reference in order to correlate the virtual CT or MRI image with the actual body section so imaged. Image-to-instrument registration at its most basic level requires some fiducials distributed in 3-dimensional space to be preoperatively imaged simultaneously with the patient. These fiducials provide an image frame of reference (ImFOR). The fiducials can either be synthetically added or consist of a set of pre-existing anatomical landmarks. There are three current methods of registering the data set of CT or MRI images of the object body segment to the actual body segment in the operating suite.
One method of registration uses CT or MRI imageable-markers or “fiducials” that can be recognized in renderings of the data set and also on the object body segment, and by touching or matching them point-to-point with a digitizing probe. Just before and during an operation, digitizing probe with sensors or emitters or reflectors for a waveform tracking system is then touched to each fiducial, enabling a computer to match the fiducials with the same points identified on the reconstructed images, planar and/or three dimensional, on a computer workstation. After a plurality of such fiducial points are matched, the computer program determines if an accurate match is obtained. This manual registration procedure locates the fiducials relative to an instrument frame of reference (InFOR). It is typical to use the operating room as the primary frame of reference (ORFOR) with the InFOR having a measured offset within the ORFOR. Thus the anatomy is registered to the image. This method is referred to as point-to-point registration.
A related registration method using fiducials attached to the patient involves mounting a reference frame directly to the patient's skeleton, such as the spine, pelvis or femur. In some instances, the skull can be fixed to a table mounted frame. The position of this frame of reference is optically tracked in real-time using the same video cameras used to track the surgical or therapeutic instrument. With the fiducials' physical location being known relative to the InFOR and the InFOR being known relative to the ORFOR and the fiducials also being known relative to the image, the location of arbitrary points in the image can be located in the physical space. Mathematically there is a bilinear transformation between the two spaces and an isomorphism exists, so operations in one space accurately reflect operations in the other. The instrument is then tracked (passive navigation) using one of several methods.
A second method of registration involves touching a segment of the body multiple times with a digitizing probe to obtain a multitude of points that will give a surface or shape that can be matched with the anatomic shape. Another version of this method uses an ultrasound probe with sensors, emitters or reflectors to map the surface of underlying bone to obtain a shape or surface. The computer program then matches the shape of the combined points to match the reconstructed image of the object part of the body. This method is referred to as shape or surface matching registration.
A third method of registration involves taking an X-ray of the body segment with a digitizing fluoroscope and matching the two-dimensional images with the three-dimensional data set.
The first registration methods require the surgeon to place fiducials on or in a patient before an imaging study and then use a digitizing probe to touch point fiducials or a surface, which is a tedious process. Using anatomic fiducials to register vertebrae for the insertion of pedicle screws has proven tedious and time consuming, so much so that this method has not gained general acceptance by orthopedic surgeons. The second method requires the exposure of a large area of bony surface in some cases, which is contradictory to one of the aims of using small incisions. The third method is more automatic but requires that a portable X-ray fluoroscopy machine be used.
Image to patient registration has been performed cutaneously by attaching spheres to a patient's scalp and then intraoperatively imaging these spheres using an ultrasonic sensor. Direct ultrasonic registration of bony tissue with their CT images is being developed.
A key component in any IGT/IGS system is the 3-dimensional (3-D) instrument tracker that informs the system computer of where the surgical or therapeutic instrument is and how it is oriented in 3-D space within the frame of reference. Currently there are four approaches to digitizing the position of the surgical or therapeutic instrument relative to some frame of reference: electromechanical; ultra-sonic; tuned, low-frequency, magnetic field transmitter source and a sensor-pointer; and infra-red optical.
An early approach to instrument tracking borrowed technology from robotic manipulators. These systems use articulated arms with optical shaft encoders or angle potentiometers to measure the angular displacements of each of the joints. Such measurements were combined to provide a mathematical estimate of the instrument's position and orientation. However, electromechanical passive articulated arms present several disadvantages that have limited their use, including: limited working volume due to constraints on arm weight; difficulties in moving free objects due to joint friction; positional accuracy limitations; the need for multiple manipulator arms in many situations; the inability to detect erroneous readings made by optical encoders at one or more joints; and difficulties associated with sterilizing or draping the large articulated arms.
Ultrasonic digitizers utilizing time-difference of arrival techniques have been used to locate instruments, but with limited success due to their: sensitivity to changes in the speed of sound; sensitivity to other operating room noises and echoes; and unacceptable accuracy in large operating volumes.
Magnetic field tracking of instruments has been tried, but suffered from operational difficulties caused by interfering fields associated with nearby metal objects and unacceptable positional accuracy for surgical or therapeutic use.